The high cost associated with manufacturing integrated circuits dictates that defective devices be diagnosed as early as possible in the manufacturing process. For this reason, it is highly advantageous to test integrated circuit devices at the wafer level before further processing and packaging. For testing, the wafers are clamped to a wafer chuck and brought into contact with highly sophisticated probes. Once probe contact is made, the circuits are tested for both functionality and power integrity. However, circuits can be tested at power levels in excess of 400 watts, which consequently generates a tremendous amount of heat build-up in the chips which is removed by the massive wafer chuck.
After wafer test, the wafers are diced into individual devices also known as chips. The good devices are mounted onto substrates to create modules. The modules then go through additional testing that may include burn in and re test.
During module test (e.g., circuit test) there is a need to accurately control temperature of high power devices. Thus the test equipment typically includes a high performance heat sink, test socket and tester electronics. The module is installed in the socket, the heat sink is brought into contact with the chip, the chip is tested, the heat sink is removed and the module is removed from the socket and sorted based on functionality.
In some applications, a mixture of water and other additives (i.e., Propylene Glycol (PG)) is placed between the chip and heat sink as a Liquid Thermal Interface (LTI) in order to improve thermal contact. Water based LTI has excellent thermal performance but has proven to be unfavorable for other reasons. For example, at high test temperatures some of the water may evaporate before the end of test. Thus, in order to ensure that water remains at the interface, special test fixtures need to be designed (i.e., new tooling and set up) in order to replenish the water during test. Otherwise, it would be necessary to disassemble the test fixture, during test, to replenish the water. In either case, costs and time are added to the test.
Also, water and other substances such as, for example, PG, is known to corrode the C4 connects. For example, the liquid can occasionally get on unprotected surfaces of the chip or carrier and result in corrosion, made worse by the voltages present. Other solutions include, permanently under fill or otherwise protect the C4s and sensitive surfaces from contact with water during the testing. However, this is undesirable because it limits the ability to salvage substrates from defective chips or to remove the good chips for sale as known good die.
Alternate interface materials have shown to each have some disadvantage. Helium is clean and non corrosive but thermal performance is insufficient. PAO (Poly Alpha Olefin) oil and various types of thermal grease have thermal performance almost as good as water but require cleaning with a solvent after test. Thermal pads have insufficient thermal performance and often leave a residue. Fluorinated fluids may be non-corrosive and clean but have poor thermal performance due to their low thermal conductivity. Liquid metals and soft metals can have very good thermal performance but can oxidize (degrade) over time and repeated reuse, they can damage the heat sinks and can have unreliable thermal contact to the chip. Greases, phase change materials and adhesives can make it difficult to separate the heat sink from the chip after test and leave a residue.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.